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Prostate cancer radiotherapy surgery. Radiologist positioning a trans-rectal ultrasound probe (scan on screen) during a treatment for prostate cancer. The ultrasound probe has been inserted into the patient's rectum, and will be used with a template grid (black square) to guide long hollow needles through the perineal skin to the prostate gland, to implant radioactive sources. This is known as brachytherapy, and is a form of radiotherapy. The radiation kills the cancerous cells.